In recent years, studies on novel devices using charge properties and spin properties of electrons at the same time are becoming more and more active. A spin transistor that is one of those novel devices uses a magnetic material as a source electrode and a drain electrode connected to each other via a semiconductor. In such a spin transistor, output characteristics can be controlled by changing the relative magnetization directions of the source electrode and the drain electrode. Specifically, the following features are utilized. When the relative magnetization directions of the source electrode and the drain electrode are substantially parallel to each other, the drain current (IDP) increases. When the relative magnetization directions are substantially antiparallel to each other, the drain current (IDAP) decreases. Where spin transistors are used in a memory or a reconfigurable logic circuit, it is necessary to increase the IDP/IDAP ratio, which is the drain current change that occurs when the relative magnetization directions are switched between a substantially parallel state and a substantially antiparallel state. At present, however, the IDP/IDAP ratio achieved via a semiconductor is not sufficiently high. Therefore, there is a demand for an improvement or a novel structure for increasing the IDP/IDAP ratio.